Polyploidization, or whole-genome duplication, refers to the acquisition of extra sets of chromosomes in a cell or organism and frequently occurs in vascular plants. Polyploidization is an essential aspect of plant evolution and can significantly modify a plant’s genetic make-up, physiology, morphology, and ecology within one or more generations [
211]. Polyploidization can affect biotic interactions and resistance to pathogens, with polyploids generally having enhanced pathogen resistance. Differences between diploids and polyploids in
R genes reflects altered pathogen resistance [
212]. For example, polyploidy can increase resistance within the gene-for-gene interactions that underlie many host–pathogen interactions and where genotype × genotype interactions are important [
213]. Quantitative resistance against
P. infestans and
Tecia solanivora in 4x potato was, moreover, observed using QTL analysis [
214]. In a previous study, neopolyploids of a monogenic resistant apple cultivar showed increased resistance to
V. inaequalis compared to diploid cultivars [
215]. Another study found that synthetic tetraploids of Livingstone potato (
Plectranthus esculentus) were more resistant to root-knot nematodes than diploids [
216]. Pathogens can also change ploidy during infections; this phenomenon occurred with
P. infestans, which caused the Great Irish Potato Famine [
217]. From the evidence available, polyploidy can induce changes in pathogen interactions and increase disease resistance by regulating genome expression, resulting in alterations in physiological characteristics, hormone biosynthesis, and improved antioxidant systems [
218], which make polyploids better competitors than diploids. For example, polyploidy was investigated in
Bremia lactucae by Fletcher et al. [
219] who reported a high incidence of heterokaryosis in
B. lactucae. Heterokaryosis has phenotypic consequences on fitness that may include an increased sporulation rate and qualitative differences in virulence.